Seeing the Subatomic

Stephen MillerSaturday Morning Physics

October 11, 2003

Particles

AtomElectrons, Protons, Neutrons

PhotonsLight Electricity & magnetism

Particles emitted as Radiation in Radioactive Decay

A Photon DetectorYour Eye

• Eye measures– Energy of photon = Color – Number of photons =

Brightness– Position = Spot on retina– Time - see objects moving

• Image processing in brain

• Photon→Chemical Reaction →Electrical Pulse

Radioactivity & Radiation

Radioactive nuclei decay

Different kinds of radiation emitted

“Alpha” particle = 2protons+2neutrons

“Beta” particle = electron

“Gamma” ray = high energy photon

Neutrons

First Radiation Detector

Ruined photographic plates

Plates stored in drawer next to uranium

Even without exposure to light, plates were fogged

Becquerel discovers radioactivity 1896

An Electron DetectorYour T.V. Screen

Electron beam causes phosphorescent screen to emit light

Thomson discovers electron(1897)

Electrons in Atoms

Electrons can occupy fixed energy levels

Electron absorbing photongets boosted to higher level

Electron can fall to lower level by emitting a photon (assuming space is available)

Phosphorescence

Phosphorescence occurs when electrons changing energy levels emit visible light (Same as fluorescent light)Electron gets boosted to a higher level by collision withParticle – electron, alpha, or gamma ray

Scintillator

• Modern detectors uses scintillating plastic or crystal

• Same principal of particle energy conversion to visible light

• Use Phototube to convert light to electrical signal

IonizationParticle knocks electron away from atom

Atom now has net positive charge (ion)

Geiger Counter

• Ionization– Electrons separated from atoms

• Electric field accelerates electrons– High voltage wire attracts

electrons

• Accelerated electrons ionize others

• Avalanche of charge created• Pulse of electrons hits wire and

makes electrical pulse

• Disadvantages:– Only counts events – No measurement of energy or

direction

Another Look at Radioactivity

• Rate of counts – radioactivity of source

• Penetration of radiation

Cloud Chamber

• Gas vapor in saturated state• Radiation ionizes atoms• Condensation forms along

path of ionization

• Shows direction of particle motion

• Shows paths of multiple particles

• Invented 1911

Cosmic Rays

Radiation from outer spacecreates shower of particlesDiscovered 1912 by Victor Hess

Cloud Chamber Discoveries

Cosmic ray studies Anti-matter

Strange particles

Heavy electrons (muons)

Anti-Matter

Positively charged “electron” called positron

Discovered 1932

photon → electron+positron “pair creation”

electron + positron → photons “annihilation”

E=mc2

Bubble Chamber

• Boiling forms along path of ionization

• Analyze photograph of bubbles • Disadvantages:

– Cannot handle high rate of events

– Data analysis time consuming

• Curvature of particles due to externally applied magnetic field

• A whole “zoo” of particles discovered

Scale of Matter

Smallest known constituents of matter are quarks and electrons

Radioactivity

Cosmic Ray Studies

Particle accelerators

Higher Energy Particles Smaller distance scales

Elementary Particles

Zoo of Particles

Combination of quarks andAnti-quarks

New kinds of quarks discoveredin collisions created at particle accelerators

Drift Chamber

• Ionization detector– Similar to Geiger counter– Thousands of sense wires

• Gives precise trajectory of particle

• Operates inside a magnetic field– curvature of the particle paths

depends on their momentum

• Instrumented for electronic readout

• Measure thousands of events/second

• Only measures charged particles

Drift Chamber

Connect the Dotsto find tracks

Energy Measurement

• Measure energy of particle • Essential for detecting Neutral

particles – neutrons don’t ionize gas in

drift chamber • Use scintillating plastic

(sandwiched between lead plates) to produce light

• The brightness of light produced in the plastic is proportional to the energy

Detecting Particles

Measure Momentum, Energy, Pattern of energy

Computer program used to find patterns

Integrated Detector

• Combine different kinds of detectors

• Usually cylindrical in shape• Inner layer is drift chamber• Followed by calorimeters• Outer layer of muon detectors

• Particle accelerator used to create new particles at center of detector

CDF Detector

Collaboration of about 500 physicists including U of M physicists – like me.

CDF Detector

Atlas Experiment

• Currently being built at CERN (Europe)

• U of M building muon detectors

• Drift tubes similar to Geiger counter

Fermilab Accelerator

Accelerators collide matter and anti-matter

CDF and D0 detectors measure results of collision

Collisions

Look for new kinds of particles

Matter & Anti-Matter annihilateEnergy converts back into new kinds of particles

Massive Particles decay in a shower of lighter particles

Top and anti-Top quarks 170x more massive than protonDecays in a billionth trillionth of a second into electrons and lighter quarks

Event

Other Questions

• How does one measure top quarks since they decay before entering the detector?– Answer given at next Saturday morning Physics lecture – Oct 18

• Besides cosmic rays which go through our bodies at a rate of 1/second, there billions of other particles going through our body each second. What are they? How do we know they are there?– Answer given at the 3rd Saturday morning Physics lecture – Oct

25

What to Remember

• Subatomic particles lose energy when colliding with electrons in atoms

• To “see” the particles simply convert this energy to another form

• Scintillation/Phosphorescence – energy converted to visible light

• Ionization – energy converted to electric pulse

• High energy particles require big detectors to capture all the energy